The present invention generally relates to facsimile communication methods and facsimile machines, and more particularly to a facsimile communication method which quickly determines a modem transmission speed and a facsimile machine which employs such a facsimile communication method.
When a call set up is made and a predetermined control procedure is completed in a facsimile communication, a modem transmission speed is determined prior to transmission of image data. Conventionally, the modem transmission speed is determined as follows. That is, a source or transmitting facsimile machine sends a training check field signal TCF to a destination or receiving facsimile machine at a predetermined transmission speed. Normally, the training check field signal TCF has a predetermined number of consecutive "0"s. The destination facsimile machine detects a number of erroneous bits of the received training check field signal TCF and returns to the source facsimile machine a confirmation to receive signal CFR or a failure to train signal FTT depending on whether or not the number of erroneous bits is less than or equal to a predetermined reference value. The erroneous bits will hereinafter be referred to as a bit error and the number of erroneous bits will be referred to as a bit error number.
Normally, the source facsimile machine sends the training check field signal TCF at a maximum transmission speed (for example, 9600 bps) and the destination facsimile machine detects the bit error number of the received training check field signal TCF. When the bit error number is less than or equal to the predetermined reference value, the destination facsimile machine returns the confirmation to receive signal CFR to the source facsimile machine and the maximum transmission speed is adopted. On the other hand, the destination facsimile machine returns the failure to train signal FTT to the source facsimile machine when the bit error number is greater than the predetermined reference value. When the source facsimile machine receives the failure to train signal FTT, the source facsimile machine retransmits the training check field signal TCF at a transmission speed which is one step slower than the maximum transmission speed. The source facsimile machine successively reduces the transmission speed of the training check field signal TCF in steps until the confirmation to receive signal CFR is received from the destination facsimile machine. Hence, the destination facsimile machine returns the confirmation to receive signal CFR and the transmission speed is determined when the bit error number of the training check field signal TCF becomes less than or equal to the predetermined reference value.
However, the modem transmission speed is conventionally determined by starting from the maximum transmission speed and successively reducing the transmission speed in steps until the bit error number of the received training check field signal TCF becomes less than or equal to the predetermined reference value. In other words, every time the bit error number of the training check field signal TCF received by the destination facsimile machine is greater than the predetermined reference value, the source facsimile reduces the transmission speed by one step and sends the training check field signal TCF at the reduced transmission speed. As a result, there are problems in that it takes a long time to determine the transmission speed when the detected bit error number of the received training check field signal TCF is relatively large and an accounting on the facsimile communication becomes expensive. Especially when the state of the line is poor and the transmission speed is slow, the signals TCF and FTT must be sent repeatedly until the transmission speed is determined, and time is unnecessarily wasted before the transmission of the image data is actually started.
The generation of the bit error in the training check field signal TCF is caused by a stationary factor or a non-stationary factor. The stationary factor includes an amplitude jitter, a phase jitter, a signal-to-noise ratio (S/N) of the communication line and the like which causes a stationary line deterioration. On the other hand, the non-stationary factor includes an amplitude hit, a phase hit, an impulse noise and the like.
When conventionally determining the transmission speed by discriminating the line quality, the discrimination also takes into account the bit error caused by the non-stationary factor. For this reason, even when the stationary line deterioration is tolerable and the stationary factor does not cause a significant bit error, the training fails if an increase of the bit error number is caused by the non-stationary factor. This means that the transmission speed must be reduced in steps until the bit error number of the received training check field signal TCF caused by the non-stationary factor becomes less than or equal to the predetermined reference value, and the bit error number must be checked every time the transmission speed is reduced. As a result, the duration and accounting of the facsimile communication increase.